In this tutorial, you’ll learn the definitions of monoprotic, diprotic, triprotic, and polyprotic acids. You will also learn about the trends of acidity between different acids, between protons within polyprotic acids, and polyprotic acid titration curves.
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Proton: In this context, a proton is a hydrogen atom whose electron has been removed (hydrogen ion). Also denoted H+.
Bronsted-Lowry acid: A proton (H+) donor.
Bronsted-Lowry base: A proton (H+) acceptor.
Ka: the equilibrium constant for an acid deprotonation reaction (in the forward direction).
What are monoprotic acids?
Most acids commonly seen are monoprotic, meaning they can only give up one proton. Examples of monoprotic acids include:
- HCl (hydrochloric acid)
- CH3COOH (acetic acid)
- HNO3 (nitric acid)
Notice that hydrochloric acid and nitric acid both have only one hydrogen in their formula, and that hydrogen is acidic (meaning it can be released as a proton). However, acetic acid has hydrogens which are NOT acidic. It is important to note that acids will often have non-acidic hydrogens.
What are polyprotic acids?
A polyprotic acid is any Bronsted-Lowry acid that can donate more than one proton. While the examples above can only release one proton, many acids have multiple acidic hydrogens. Here are some examples:
- H2SO4 (sulfuric acid)
- H3PO4 (phosphoric acid)
- C10H16N2O8 (Ethylenediaminetetraacetic acid, or EDTA for short)
Each of these acids have more than one acidic proton. Sulfuric acid has two, so it would be called a diprotic acid. Phosphoric acid has three, so it’s called a triprotic acid. EDTA has four acidic protons, meaning it would technically be called a tetraprotic acid, but in practice any acid with more than three protons is just called polyprotic. In theory, there is no limit to the number of acidic protons a polyprotic acid could have.
How do polyprotic acids work?
Recall that when an acid gives up a proton, it forms what is called the conjugate base. For monoprotic acids, the conjugate base has no acidic protons and can only act (unsurprisingly) as a base. For polyprotic acids however, the conjugate base of the original acid DOES have acidic protons – in other words, the conjugate base is also an acid. Let’s look at some examples.
HCl ⇌ Cl– + H+
In the equilibrium above, hydrochloric acid, a strong monoprotic acid, gives up its acidic hydrogen. This forms the conjugate base, chloride, plus a proton. There are no more acidic protons, so there is no further reaction.
H3PO4 ⇌ H2PO4– + H+
The equilibrium above shows the loss of the first acidic proton from phosphoric acid. Note that after this proton is lost, there are still acidic protons left on the conjugate base. This means that the conjugate base can act as an acid in another equilbrium:
H2PO4– ⇌ HPO42- + H+
And once again, there is still an acidic proton left on the (second) conjugate base. Thus there is one last deprotonation that could occur:
HPO42- ⇌ PO43-+ H+
There were three deprotonations that just occurred, because the original molecule, phosphoric acid, is triprotic.
In a triprotic acid, which Ka has the highest value?
The acidity of polyprotic acids is simple: each proton is less acidic than the last. This can be represented numerically by the Ka or pKa values of each subsequent species. Recall that Ka is the equilibrium constant for an acid-base reaction: the greater it is, the stronger the acid.
H3PO4 ⇌ H2PO4– + H+ Ka1 = 6.9 x 10-3
H2PO4– ⇌ HPO42-+ H+ Ka2 = 6.2 x 10-8
HPO42- ⇌ PO43-+ H+ Ka3 = 4.8 x 10-13
Between the first and second deprotonations, the Ka drops by nearly 5 orders of magnitude! That means the first proton was 100,000 times more acidic than the second. Between the second and third deprotonations, the Ka dropped by MORE than 5 orders of magnitude. This means that phosphoric acid’s first proton is about 10 billion times more acidic than its third. It’s generally somewhat difficult to remove the last proton from a diprotic or triprotic acid.
This table shows a variety of acids, including many polyprotic ones. Feel free to scan through, and notice that every polyprotic acid has a greater Ka1 than Ka2, which is greater than Ka3, et cetera. If you choose to use pKa instead, remember that higher pKa value represents lower acidity, the opposite of Ka.
Are diprotic acids stronger than monoprotic acids?
Not necessarily. The number of acidic hydrogens in a molecule has nothing to do with how acidic those hydrogens are (or specifically, how acidic the MOST acidic one is). Hydrochloric acid (monoprotic) is much stronger than phosphoric acid (triprotic), and sulfuric acid (diprotic) is much stronger than hydrofluoric acid (monoprotic). Basically, the number of hydrogens doesn’t make a difference.
Polyprotic acid titration curve
We learned before that when a polyprotic acid loses its first proton, it forms a new weak acid. This is important to remember when carrying out a titration, arguably the most common and important technique in acid/base chemistry. Because the newly-formed acid (the conjugate base of the original acid) can lose its protons, titration curves for polyprotic acids look like several “normal” (monoprotic) titration curves attached to each other, one for each acidic proton. Consider the curve below, representing a diprotic acid (such as sulfuric acid) being titrated with a strong base.
There are four characteristic points in this curve, labelled A, B, C, and D.
At point A, 0.5 moles of base have been added, meaning half of the diprotic acid has been deprotonated. Does this mean that half the acid doesn’t have any protons left? NO! It means half of the acid has lost one proton, and still has one left. The other half has both of its protons. At this point, the solution is a buffer. See the linked article to learn what a buffer solution is. The solution is a buffer because half of original acid has been converted to its conjugate base. Point A can be called a buffering region, specifically the first buffering region, as there will be a second.
At point B, 1 mole of strong base has been added. Recall that the first proton is always more acidic than the second. This means that ALL of the original acid has lost its first proton and ONLY its first proton. Point B is considered to be an equivalence point. Unlike a normal titration, with only one equivalence point, titrations of polyprotic acids will have several, each corresponding to the complete loss of each successive acidic proton.
At this point, half of the analyte has lost both of its protons, and half still has one left. Because it’s a 50/50 mixture of acid and conjugate base, it’s considered a buffer.
Point D is the second (and final) equivalence point. At Point D, all of the original acid has been fully deprotonated and converted to a base.